1. Field of the Invention
The present invention relates to methods for obtaining metallic iron by the reduction of iron oxides, such as iron ores, together with carbonaceous reducing agents. In particular, the present invention relates to a method and an apparatus for producing high purity metallic iron by efficiently separating slag ingredients from raw materials containing iron oxides.
2. Description of Related Art
Conventional methods of making iron by direct reduction of iron oxides in iron ores or iron oxide pellets using a carbon material or a reducing gas include the shaft furnace method typically represented by the Midrex method. This direct iron making method comprises blowing a reducing gas, produced from, e.g., natural gas, from a tuyere at the bottom of a shaft furnace to reduce iron oxides and obtain reduced iron. More recently, methods of making reduced iron using carbon material such as coal as a reducing agent instead of natural gas, such as the SL/RN method, have been put into practical use.
U.S. Pat. No. 3,443,931 discloses another method of making reduced iron in which powders of a carbon material and iron oxides are mixed and formed into pellets prior to reduction on a rotary hearth.
Reduced iron made by these conventional methods can be charged, either directly or after being formed into pellets or briquettes, into an electric furnace and used as an iron source.
As recycling of iron scraps has developed in recent years, it has been noted that the reduced iron obtained by the methods described above can act as a diluent for impurity elements present in the scraps.
However, because conventional methods of making reduced iron fail to remove large amounts of slag ingredients such as SiO.sub.2, Al.sub.2 0.sub.3 and CaO that are present in iron oxide and carbon material starting materials (e.g., gangue in iron ores, ashes in the coal), the metallic iron produced is of lower iron quality (i.e. lower purity).
In practice, slag ingredients are separated and removed from metallic iron in later refining steps. However, the presence of large amounts of slag in reduced iron not only lowers the yield of refined molten metal but also raises the cost of operation of the electric furnace.
As a result, a demand has developed for reduced iron of high iron quality containing fewer slag ingredients. However, in order for conventional reduced iron making methods to satisfy the demand, iron ores of high iron quality have to be used as the starting material for making the reduced iron. In practice, this has greatly narrowed the range of iron oxide starting materials that can be used to make iron.
In the conventional methods described above, it is necessary to mix the iron oxide source and the carbon material, and to mold the mixture preliminarily into lumps or pellets using a binder or by sintering. This preliminary molding inevitably increases the burden on the facility and operation.
Japanese Patent Laid-Open Hei 9-256017, filed by the present applicant, discloses a method comprising preliminarily molding a mixed powder of a carbonaceous reducing agent and iron oxide into a spherical or pellet-shaped molded product; heat reducing the molded product to form and grow a metallic iron shell on the outer surface of the molded product; and increasing the reduction potential in the metallic iron shell. The method is capable of efficiently reducing the iron oxide on the inside of the molded product, separating the resulting metallic iron and slag, and making high purity metallic iron.
However, even in this method the starting mixture has to be preliminarily molded into a spherical or pellet shape using a binder or by sintering, increasing the burden on the facility and the operation.
Japanese Patent Laid-Open Hei 8-27507 discloses another direct reduction method of making iron that includes stacking layers of powdery iron oxide and a carbonaceous reducing powder such as coal containing a desulfurizer on a movable hearth and heating the layers to form a sponge iron. The reference emphasizes that since the iron oxide is reduced by the carbonaceous reducing agent, and sulfur ingredients contained in the carbonaceous reducing agent are captured by the desulfurizer, sponge iron with less sulfur content can be obtained according to the method and subsequent desulfurizing loads can be moderated.
While this method does not require preliminarily molding iron oxide starting material into pellets, the reduction efficiency of the method is low because the iron oxide source and the carbonaceous reducing agent are not in direct contact with each other. Thus, long periods of time are required for heat reduction. Because of low productivity, the method is not practical for use on an industrial scale.
In addition, because the method produces reduced iron in the form of sponge iron, and a large amount of gangue ingredients are present in sponge iron, the quality of the reduced iron is relatively low. If reduced iron of such low iron quality is supplied as an iron source to an electric furnace, undesirable effects on the operability of the electric furnace result from the increase in the amount of slag that forms. In addition, various other problems result such as a reduction in iron yield due to the loss of iron into the slag, an increase in energy consumption, and a decrease in productivity. Since these problems become more pronounced as the iron content in an iron oxide source is decreased, it is almost impossible to use an iron or iron oxide source of low quality as a starting material in practice. Instead, only an iron oxide source of high quality can be used.
The present invention has been accomplished in view of the foregoing problems in the prior art.
An object of the present invention is to provide a method and an apparatus capable of making reduced iron of high Fe purity, containing relatively small amounts of slag ingredients, from iron oxide sources of both high and low quality without requiring preliminary molding of starting materials into lumps or pellets.